Optical networks are using polarization multiplexed coherent modulation schemes to accommodate the ever-increasing bandwidth demands. An optical link is formed between two nodes or network elements via optical modems (also referred to as optical transceivers, transponders, transmitters/receivers, etc.).
Modems are typically configured to provide sufficient operating margin in order to guarantee a given signal quality over a specified interval such as the anticipated lifespan of the device. In many cases these performance guarantees are expressed in terms of a maximum allowable Bit Error Rate (BER) at the output of a receiver's Forward Error Correction (FEC) circuit or post FEC BER. There is a strong relation between the signal to noise ratio and the maximum rate of data transmission for a given bit error rate. The available Signal-to-Noise (SNR) margin is the difference between the SNR which the modem is operating at and the SNR below which an unacceptable number of bit errors are expected. The SNR decreases as noise increases where noise comes from a variety of sources internal to the transceiver such as the quiescent noise from feedback loops and external to the transceiver such as ASE from optical amplifiers in the link. There are also transitory noise sources such as noise arising from polarization, laser and clock transients. For a given reach and data rate the transceiver must be configured to insure that there is sufficient margin to satisfy the performance guarantees for the combinations of noise sources which are anticipated over the lifetime of the guarantee.
An aspect of optical modems is they can be configured in various different operating modes to address various aspects of the operation, such as State of Polarization (SOP) tracking, bias control, laser transients, and the like. As described herein, the operating modes generally refer to some settings in the optical modem for the operation of feedback loops, control loops, etc. to maintain signal quality. There is also a trade-off between operating modes and margin (dBQ) which leads to the question of what is the optimal setting for the operating modes at any given time based on current operating conditions.
For example, polarization multiplexed modulation schemes require an optical receiver that is able to recover the SOP of a signal and to track any changes which occur during the lifetime of the channel. The rate of change of SOP of the signal into the optical receiver can vary by orders of magnitude depending on the link through which the channel is propagated. The conventional methodology is to provision the SOP tracking circuit at the Start of Life (SOL) based on the maximum expected rates of change of SOP over the life of the transceiver. Increasing the maximum polarization tracking rate of the receiver often comes at the expense of higher quiescent noise from the tracking circuit, which directly reduces the available SNR margin. This set and forget approach with a static provisioning of an optical modem is suboptimal for cases where transient events are not uniformly distributed. For example, cases have been observed where polarization transients caused by lighting have a mean arrival time that is orders of magnitude larger than the median. In this case transients arrive in bunches, and static provisioning operates the tracking circuit with excess bandwidth (noise) during the time between bunches when transients are unlikely to occur. For example, SOP transient arrival statistics are described in Douglas Charlton et al., “Field measurements of SOP transients in OPGW, with time and location correlation to lightning strikes,” Opt. Express 25, 9689-9696 (2017). Also, conventional optical modems can include hardware to detect and localize SOP transients (e.g., integrated polarimeters or the like). An example is described in commonly-assigned U.S. Pat. No. 9,774,392, issued Sep. 26, 2017, and entitled “SYSTEMS AND METHODS USING A POLARIMETER TO LOCALIZE STATE OF POLARIZATION TRANSITS ON OPTICAL FIBERS,” the contents of which are incorporated by reference herein.